No. 1558: Insanity

Disembodied speech balloon: I'll be going insane any second now!
Garfield: How will you be able to tell?

The author writes:

I came up with this while I was working on a different strip in which I had a different punchline in mind. You may or may not have seen the other strip depending on which of these was published first. While removing Jon from the original, instead of the gag I had in mind (which I won't give away), I decided it worked just as well without anybody at all. I think this strip sort of implies that Garfield himself is going insane. So here it is, salvaged from my original editing of the strip.